1. Field of the Invention
The present invention relates to an exhaust manifold made of steel of the welding construction, which is used for an internal combustion engine.
2. Description of the Prior Art
Examples of the conventional exhaust manifold made of steel of the welding construction are as follows (refer to Japanese Utility Model Provisional Publication No. 62-108516).
There is an exhaust manifold, as shown in FIG. 7, in which a main pie 1 or a branch pipe 2 and a flange 3 as well as the main pipe 1 and the branch pipe 2 are connected with each other all over the circumference by permanent welding. As to the way of connection, two parts are welded and jointed by parmanent welding beads 11 and 12 as shown in FIGS. 8 and 9.
Further, there is another exhaust manifold having spot welding as temporary one in addition to the above-mentioned parmanent welding as shown in FIG. 10. Two ways of spot welding can be adopted: one in which, as shown in FIG. 11, a spot welding bead 13 is located at a different point than that of a permanent welding bead 11 and thereafter permanent welding is carried out; and the other in which, as shown in FIG. 12, a permanent welding bead 12 is applied on a spot welding bead 14.
A primary object of the execution of spot welding is to prevent deterioration of assembly precision and/or displacement of assembly disportion caused by thermal stress of welding in case only permanent welding is carried out (FIGS. 7-9) and/or to make in part a contribution to rigidity on use.
In regard to permanent welding bead material, it is to be noted that, when materials having the same metallic crystal structure "common material" are used for the main pie 1, the branch pipe 2 and the flange 3, the common material is naturally adopted in consideration of countermeasure against producing thermal fatigue crack. Further, the common material is generally used as spot welding bead material.
However, in the above-mentioned conventional exhaust manifold made of steel of welding construction, the common material of the main pie 1, the branch pipe 2 and the flange 3 is used in spot welding as temporary welding, so that, as shown in FIG. 13, if crack is produced at the spot welding bead 13 on use, the crack will develop from the point 0 to the main pipe 1 or the branch pipe 2 and grow to a through hole after a while. Thus, exhaust gas passing through an exhaust passage 31 will leak in the air resulting in deterioration of engine performance and endurance.